Equipment for separation of multiple fluid phases has, in the past, utilized one of four principles for the separation of the constituent elements of an emulsion: (a) natural coalescence of the phases in a large, quiet vessel; (b) coalescence by the use of a large surface area, for example, inclined plate, wire mesh, plastic media, etc. (c) air flotation, which uses a large vertical column in which the dissolved air in a high pressure water stream is injected into a stream of multiple fluid phases near the lower end of the column and the effervescent bubbles carry the oil to the surface; and (d) application of centrifugal force using such equipment as high speed centrifuges and hydrocyclones.
The aforementioned methods of fluid phase separation have had varying degrees of success in effecting a complete separation of the two phases from the emulsion. Some of the methods are successful, if, for example, they are given enough time or surface area. All of the apparatuses that apply the above methods have at least one or more of the following disadvantages: a failure to produce a low enough concentration of oil in the processed water; a bulky size, large footprint, or excessive weight; a limitation on the effective oil droplet size removal from the water; and easy fouling of the equipment by particulates, asphalts, paraffins, etc.; an inability to tolerate the use of centrifugal pumps to move the emulsion; and an inability to tolerate large fluctuations in the feed supply.
Those apparatuses of the prior art that employ natural coalescence use either open or closed vessels with baffles to inhibit mixing and turbulence, and require long residence times (hours, days, or even months) or chemical additions to promote complete separation of the fluid phases. The cost of tankage or chemicals may be cost prohibitive.
The use of certain surfaces have been found to enhance coalescence of oil/water emulsions. Such surfaces are made from materials which attract the noncontinuous phase preferentially. Large surface area plates, meshes, and packing materials have been incorporated into coalescence vessels to decrease volume and residence times, the effectiveness being in direct proportion to the size of the droplets as compared to the probability of the noncontinuous phase contacting one of the surfaces. By this method of coalescence, the presence of extremely small droplets of a finely dispersed immiscible phase results in significantly higher effluent levels.
Air flotation is an alternate method for generating large surface areas on which the oil may coalesce and be floated to the top of a vessel. In order for air flotation to be effective, a gas must be dissolved into the continuous phase liquid at elevated pressure and then released to form the tiny bubbles at an even distribution throughout the liquid. This usually requires recycling some of the continuous phase back to the beginning of the separation process. If the noncontinuous phase becomes too concentrated, the recycle stream becomes excessive. For the case of air flotation, very fine emulsions or very low surface tension organics may not be effectively separated. In practice, the equipment loses efficiency very quickly and must be cleaned frequently.
Centrifugal separators utilize the density difference between liquid phases to enhance separation. A high speed centrifuge will magnify this density difference with increased rpm's. Separation of an oil/water emulsion is effective as long as the droplet size is sufficient to overcome the Brownian motion forces, which tend to suspend and circulate the droplets, preventing the droplets from coalescing. Hydrocyclones use similar principles except that the rotational motion is imparted by tangential inlet rather than vessel rotation. In both cases, the light phase is removed at the center and the heavy phase at the periphery of the vessel. The centrifuges often have nonporous disks rotating in the vessel to promote the separation through increased surface area.
Centrifugal disk filters such as are disclosed in U.S. Pat. Nos. 3,997,447 and 4,698,156 issued to Breton et al. and Bumpers, respectively, have typically used rapidly rotating filter elements with axial removal of filtrate and containment within a pressure vessel in which a suspension of solid particles are injected to effect separation. Such disk filters are not suitable as designed to continuous separation and removal of the two phases. The lighter phase in the Breton et al. or Bumpers design tends to accumulate near the shaft and either blind the surface or bleed through the disks into the filtrate stream.